The present invention relates to a method for representing complex colors by ink dot-patterns and more particularly to a method for producing, by using an ink-jet color printer, color images with complex colors such as a color photograph.
In ink-jet color printers in general use, a plurality of colored inks, for instance at least cyan, magenta and yellow, are ejected to paint a color image in the form of an ink dot-pattern. These ink-jet color printers have used a method in which an image with half-tones is represented by controlling the quantity of ink drops to be deposited on dot matrices provided one for each of the picture elements on a recording paper, and an image with complex colors is represented by mixing different colors of ink drops.
In a method for representing complex colors, for instance as disclosed in Japanese patent unexamined publication No. 54-126416, or our U.S. Ser. No. 317,850, filed Nov. 3, 1981 and entitled "Ink-Jet Color Printing Apparatus", ink-jet heads provided one for each color of ink and arranged along a scanning direction deposit ink drops of different colors on a single scanning line (that is, a picture element line) so as to superimpose individual color inks on the given image point at the correct time. This is effected with the aid of digital timer timers that delay the picture signals that act on one head by a time interval corresponding to the distance between that one head and the preceding head. The method described above, wherein a plurality of ink drops having different colors are laid all in one spot, can be considered as a kind of subtractive color mixture. However, in this kind of subtractive color mixture, it is hardly possible to represent vivid complex colors because of the resulting turbid mixture of ink drops having different colors.
For representing vivid complex colors without the ink drops being turbidly mixed, it is desirable to deposit several colors of ink drops in side-by-side arrangement in a dot matrix in such manner that no ink drop overlaps an ink drop previously deposited. This side-by-side arrangement of ink drops is quite similar to an additive color mixture in effect and hence can be classified as a kind of additive color mixture technique. In an actual ink-jet printer using several different colors of ink, although the superimposition of different color ink drops unavoidably occurs in a plurality of cells, the number of cells in which different color ink drops are deposited to overlap one another or the amount of ink in these cells is less than that of the remaining cells in which a single ink drop is deposited, so that a method using the side-by-side arrangement of ink drops in a dot matrix as aforementioned can produce an effect quite similar to that of an additive color mixture. Therefore a method using a side-by-side arrangement of ink dots constituted only by a plurality of single ink dots or including a small number of ink dots comprising different color ink drops is hereinafter called providing a mean-value color mixture. However, in such a method of providing a mean-value color mixture there is the disadvantage that, when representing complex colors of high density levels, a group of ink dots forming a single picture element are liable to change the appearance of the color.
Various techniques for producing half-tones used with an ink-jet color printer have been developed, wherein it has been proposed, as in U.S. Pat. No. 3,977,077, to vary the number of ink dots in a dot matrix and to provide a pattern of arrangement in the dot matrix having m possible positions in the row and n possible positions in the column, that is, m.times.n cells (m and n being integers) for one picture element so as to paint images with half-tones in a sufficiently large number of steps of gradation, for instance in at least 16 steps of gradation. A method of this kind, however, has the disadvantage that delicate differences in half-tones are hardly expressible with only a restricted number of steps of gradation, and an increase in the number of steps unavoidably requires the provision of a large number of cells of a dot matrix for one picture element.